Some publications disclose adhesive compositions which control the adhesion properties as desired by the use of a tackifying polymer having pressure-sensitive adhesion properties, and a crystalline polymer in combination.
For example, U.S. Pat. No. 5,192,612 (=JP-B-3021646) discloses a pressure-sensitive composition comprising a pressure-sensitive adhesive base resin (a tackifying polymer such as an acrylic polymer), a detackifying resin and a detackifying particulate. A specific preferable example of the detackifying resin is substantially linear polycaprolactone having a molecular weight of about 3,000 to about 342,000. Polycaprolactone is a crystalline polymer which exhibits non-tackifying properties at room temperature (about 15 to 30° C.)
The above-described pressure-sensitive adhesive can be adhered to an adherent by pressing the adhesive against the adherent. The above detackifying resin and the detackifying particles effectively reduce tack on the surface of the adhesive at room temperature and improves repeelability. The repeelability means that an adhesive material can be adhered, peeled, readhered and repeeled, and the positioning or the position-adjustment is easy prior to the final adhesion. In this art field, the repeeling properties are also known as “repositionability”, “slidability” (ease of sliding) or the like. The amount of the pressure sensitive adhesive base resin contained in the above pressure sensitive adhesive is preferably from 55 to 98 wt. %, while that of the detackifying resin is preferably from 1 to 30 wt. %, based on the entire weight of the adhesive. However, in the above US patent, it is not supposed that the adhesive is peeled off during or after the use, once the adhesive is finally adhered to the adherent.
The pressure sensitive adhesive disclosed in U.S. Pat. No. 5,412,035 (=JP-A-6-510548) is a pressure sensitive adhesive composition which becomes pressure sensitive at least at one temperature in the range between 20° C. and 40° C., and comprises (1) at least 50 wt. % of a polymeric pressure-sensitive adhesive component and (2) a crystalline polymer in an amount of larger than 0 wt. % and less than 50 wt. %, each based on the weight of the whole solid components. The crystalline polymer is usually non-tacky at room temperature and intimately mixed with the polymeric pressure-sensitive adhesive component.
The melting point Ta (° C.) of the crystalline polymer, which is measured in the composition, is lower than the melting point Tm (° C.) of the crystalline polymer as such, and the difference Tm−Ta is preferably from 1° C. to 9° C. In the above US patent, Tm is preferably from 20° C. to 102° C. The disclosed adhesive composition is an adhesive having thermal repeelability, that is, a peel strength P2 (g/cm) at a certain temperature higher than Ta is smaller than a peel strength P1 (g/cm) at a certain temperature lower than Ta. Such an adhesive may be called a “thermal-peeling-easy” adhesive. The above peel strength P1 is measured at a temperature T1 in the range between (Ta-10) and (Ta-4), while the peel strength P2 is measured at a temperature in the range between (Ta+4) and (Ta+10).
That is, U.S. Pat. No. 5,412,035 teaches that the thermal-peeling-easy adhesive, which is be easily peeled off during or after the use once the adhesive is finally adhered to the adherent, can be produced through the optimum combination of the polymeric pressure-sensitive adhesive component and the crystalline polymer which is intimately mixed with the polymeric pressure-sensitive adhesive component. This US patent specification or the corresponding JP-A publication does not disclose that polycaprolactone may be used as a crystalline polymer.
JP-A-2000-119624 discloses a heat-activation adhesive comprising a specific tackifying polymer and a polyester such as polycaprolactone, etc. Using the disclosed adhesive, an article such as an electronic part or a polymer film can be adhered to an adherent by heat pressing (pressing after heating or pressing while heating). Polycaprolactone efficiently functions as a thermoplastic polymer, and the combination of polycaprolactone and the tackifying polymer having good compatibility with polycaprolactone can form a heat-activation adhesive having improved adhesion strength.
It is essential for the disclosed tackifying polymer to have two functional groups, that is, a hydroxyl group and a phenyl group, in the molecule, and the compatibility with polycaprolactone is improved by the functions of these functional groups.
With the above-described thermal-peeling-easy adhesive disclosed in U.S. Pat. No. 5,412,035, an adhered article (e.g. an adhesive sheet comprising a substrate fixed to an adhesive layer containing an adhesive, etc.) can be relatively easily peeled off from an adherent, after it is heated to a specific temperature, usually, a temperature higher than the melting point of the crystalline polymer. However, the conventional thermal-peeling-easy adhesives may be insufficient in the following properties, and thus their improvements are necessary:
(i) The easily peelable state is maintained for a specific period of time (to prolong the easily peeling time),
(ii) The adhered article can be peeled off cleanly without leaving the adhesive (the polymeric components such as the tackifying polymer) on the adherent (to avoid so-called “adhesive leavings”).
With the conventional adhesives, the thermal-peeling-easy effect is relatively quickly lost when the temperature of the adherent becomes lower than a certain specific temperature. For example, the adhesives recover the peel strength of substantially the same level as that prior to heating only within 2 to 3 minutes after heating.
For instance, when such an adhesive is used as the adhesive layer of an adhesive sheet to be used outdoors (e.g. exterior decorative sheets or reflection sheets for signs, etc.), and the adhesive sheet is peeled off under relatively low temperature conditions such as winter seasons, the temperature of the adherent is spontaneously lowered to a temperature less than the specific temperature before the entire sheet is peeled off.
When such as adhesive is used as the adhesive layer of an adhesive sheet having a relatively large area (usually 400 cm2 or larger), the following problem may arise:
When a certain part of the adherent is heated through the adhesive sheet, and then other part of the adherent is heated, the once heated certain part starts to cool while the other part is heated. Even when the entire adhesive sheet and the adherent are uniformly heated, one part of the adhesive sheet start to cool while other part of the adhesive sheet is being peeled off.
When the above adhesive is used to adhere relatively small parts (e.g. electronic parts, etc.) to adherents (e.g. other electronic parts), the following problem may arise:
When the relatively small articles are peeled off and separated from the adherents, it is efficient to heat a plurality of the parts in an oven or the like, take them out from the oven and then remove each part from the adherent. However, if the number of the parts is large, the remaining parts are cooled while some parts are being removed from the adherent.
In any case, when the peelable time is short, the peeling work becomes difficult.
Sometimes, the articles such as the adhesive sheets or the parts should be removed from the adherents, after they are used for a relatively long time (several months or longer). When the articles are removed from the adherents after the long time use, the adhesive layer is cohesively failed, so that the adhesive layer is often left on the adherent. Such adhesive leavings should be prevented in any applications.
However, none of the above patent specifications and publications disclose any method to solve the above problems (i) and (ii).
Furthermore, U.S. Pat. No. 5,412,035 does not disclose a thermal-peeling-easy adhesive which can be readhered after peeling. Readhesion properties are important properties when the adherents are changed, or the position of the adhesion on the adherent surface is changed. In such applications, adhesives, which can be easily readhered only by pressing, are very advantageous.
Thus, one object of the present invention is to provide a thermal-peeling-easy adhesive composition, (1) which achieves the easily peelable state by heating it to a specific temperature to decrease the peel strength to a value smaller than that prior to heating, on a desired occasion after it is adhered to an adherent, (2) which can maintain such an easily peelable state for a certain period of time, (3) which can be peeled off without leaving the adhesive on the adherent, and (4) which can be easily readhered to an adherent (including other adherent) after peeling.